The present invention relates generally to an epicyclic gear train such as for use in rotary wing aircraft, and in particular, to a high-ratio epicyclic gear transmission incorporating idler sun and planet gears to provide an increased load carrying capability.
Rotary wing aircraft typically employ one or more high-speed turbine engines to drive a rotor providing lift and propulsion. A main gear transmission between the engine and the rotor is necessary to transmit engine power to the rotor while reducing the input engine speed to the appropriate output rotor speed. The main gear transmission is usually the heaviest subsystem in the drive train of the aircraft. Increasing power throughput and reducing the weight of the transmission is very desirable for modern rotary wing aircraft.
Owning to favorable power density values, epicyclic gear trains are the most popular gear system for the use in aircraft drive trains, being used virtually in all rotary wing aircraft main rotor drive transmission. A typical transmission configuration of a simple epicyclic gear train consists of a sun gear, a co-axially arranged ring gear, a planet carrier, and a set of planet gears (typically between three and six) supported on the planet carrier. The planet gears are spaced in the annular space between the sun gear and the ring gear. The sun gear is driven by an input shaft of the gear train and the planet carrier is operatively connected to an output shaft of the gear train. The ring gear is usually fixed to a transmission housing. The construction of the epicyclic gear train provides multiple load paths from the input shaft through the multiple planets to the output shaft, and the load is shared among the planet gears. However, as the speed ratio increases, the number of planet gears that can be packed into the epicyclic gear trains is limited, thereby limiting the load sharing capabilities.
Compound epicyclic gear trains have been utilized to improve the speed ratio of a simple epicyclic gear trains, such as shown in U.S. Pat. No. 6,966,865 to Drago et al., which discloses an epicyclic gear train employing multiple planet clusters. Each planet cluster in the compound epicyclic gear train includes a planet gear shaft, a large planet gear, and two small planet gears. The large planet gear and the two small planet gears are mounted on, and rotate with, the same planet gear shaft. The large planet gear meshes with a sun gear and the small planet gears each mesh with a fixed ring gear which includes two separate rings. To incorporate the maximum possible number of planet clusters into the gear train, two sets of planet clusters are employed. Each planet clusters is arranged with the large planets staggered axially, meshing with the sun gear, or with tilted-axis planet clusters, as shown in U.S. Pat. No. 7,507,180 to Robuck.
While such simple and compound epicyclic gear trains are suitable for use in the drive trains of rotary wing aircraft, further improvement in power density values for the drive trains is very desirable, as a reduction in weight and package size provides additional advantages over current epicyclic gear trains. Accordingly, it would be advantageous to provide an improved high-ratio epicyclical gear train having an increased load carrying capacity.